Presenilin 1 and 2 (PS1 and PS2) are highly homologous proteins implicated in familial Alzheimer's disease (FAD) (Rogaev et al., 1995; Sherrington et al., 1995). All FAD-mutations in the presenilins (PSs) increase the secretion of the highly amyloidogenic Aβ42 peptide, a major constituent of the plaques in the brains of Alzheimer's Disease (AD) patients (Citron et al., 1997; Duff et al., 1996; Scheuner et al., 1996). Aβ42- and the more abundant Aβ40-peptide are generated from the larger amyloid precursor protein (APP) by the consecutive action of two enzymes, β- and γ-secretases (De Strooper and Annaert, 2000; Haass and Selkoe, 1993; Selkoe, 1998). Several lines of evidence imply the PSs in γ-secretase activity. (i) Aβ secretion in cell lines and neurons derived from PS1 −/− or PS1 −/− PS2 −/− embryos is strongly inhibited while the α- and β-cleaved APP C-terminal stubs, the immediate substrates for γ-secretase, accumulate in these cells (De Strooper et al., 1998; Herreman et al., 2000; Naruse et al., 1998; Zhang et al., 2000). (ii) PSs are part of a multiprotein complex that exhibits γ-secretase activity in detergent extracts (Li et al., 2000). (iii) Indirect evidence implies two conserved aspartic acid residues of the transmembrane domains 6 and 7 in the catalytic activity of the γ-secretase (Wolfe et al., 1999). This putative active site displays remote similarity with the catalytic site of the bacterial type-4 prepilin peptidases (Steiner et al., 2000). (iv) Potent γ-secretase inhibitors designed to act as transition state analogues bind PSs (Esler et al., 2000; Li et al, 2000).
PSs are also required for the regulated intramembrane proteolysis of the Notch proteins (De Strooper et al., 1999; Struhl and Greenwald, 1999), thereby acting as molecular switches between proteolysis and cell signaling (Annaert and De Strooper, 1999; Brown et al., 2000). While the absolute requirement of PS for γ-secretase processing is thus clearly established, several observations indicate that a “PS is γ-secretase” hypothesis is probably too simplistic. PSs are, for instance, integrated into a multiprotein complex (Capell et al., 1998; Verdile et al., 2000; Yu et al., 1998) and one of its components, nicastrin, is apparently involved in the regulation of its proteolytic activity (Yu et al., 2000). Other observations also indicate that the exact role of PS in γ-secretase activity needs further scrutiny. For instance, the mutation of Asp257, one of the two aspartates of the putative catalytic site of PS, as well as certain other missense mutations in PS, inhibit Notch but not APP cleavage (Capell et al., 2000; Kulic et al., 2000). This is difficult to conciliate with the idea that the two aspartates constitute the active site of a single protease. Another paradox that needs further work is the discrepancy between the subcellular distribution of PSs and the sites where γ-secretase cleavage of APP or Notch is supposed to occur (“the spatial paradox”: Annaert and De Strooper, 1999; Annaert et al., 1999). The complexity of the issues involved is illustrated by studies that demonstrate the role of PS in the Wnt/β-catenin signaling pathway. Several authors found that PS can bind proteins of the armadillo family. PS1 indirectly modulates Wnt signaling by stabilizing β-catenin (De Strooper and Annaert, 2001; Kang et al., 1999; Nishimura et al., 1999; Soriano et al., 2001; Zhang et al., 1998). As β-catenin binding to PS1 is independent of γ-secretase function (Saura et al., 2000) it follows that PS1 contains several functional domains, and regulates at least more than one signaling pathway. The exact molecular domains involved in the interaction between presenilin and its substrates are not known. In the current invention we have identified that presenilin is binding in a complex two-part to type I transmembrane regions. These novel binding regions are important targets for drug development for the modulation of presenilin mediated intramembrane cleavage and can be used, for example, for drug development in the fight against Alzheimer's disease. It has been shown in the art that presenilins are endoproteolysed yielding saturable and stable complexes of N-terminal and C-terminal fragments (NTF and CTF) (Thinakaran et al., 1996; Thinakaran et al., 1997) and that the integrity of PS including intramolecular interactions between both fragments, is required for its normal biological function (Saura et al., 1999; Tomita et al., 1998). It has also been shown that co-immunoprecipitation of Notch with the PS1-NTF as well as with the PS1-CTF can occur (Ray et al., 1999), although the exact binding sites were not identified. Furthermore mutational analysis has demonstrated that the C-terminus of PS is needed for the stabilization, endoproteolysis and Aβ42 overproduction caused by FAD-linked mutations (Thinakaran et al., 1997; Tomita et al., 1999).